It's like, there's this chain of command that Stevens is sticking to. The "guys who's been there". I hate that kind of thinking. At some point this season, you have to throw all of that out the window and get these rooks ready for year 2.
More minutes to the young guys, means, by definition, less for the vets. Stevens doesn't have the courage to cut their minutes. Hence, the status quo will be preserved until Ainge makes a move.